Ethylene and other alkenes (also commonly referred to as olefins) are important commodity chemicals and are useful starting materials for numerous chemical products, including polymeric products, such as polyethylene. Traditionally, alkenes, such as ethylene, have been produced by steam or catalytic cracking of hydrocarbons derived from crude oil. However, as crude oil is a finite resource, there is interest in finding alternative, economically viable, methods for producing alkenes, in particular ethylene, which can use feedstocks not derived from crude oil.
In recent years the search for alternative materials for alkene production has led to the production of alkenes by the dehydration of alcohols, such as methanol and ethanol, which can be produced by the fermentation of, for example, sugars, starches and/or cellulosic materials, or alternatively may be produced from synthesis gas.
Examples of processes for the preparation of alkenes from alcohols include:
U.S. Pat. No. 5,817,906 discloses a process for producing light olefin(s) from a crude oxygenate feedstock comprising alcohol and water. The process employs two reaction stages. Firstly, the alcohol is converted, using reaction with distillation, to an ether. The ether is then subsequently passed to an oxygenate conversion zone containing a metalaluminosilicate catalyst to produce a light olefin stream.
EP 1792885 discloses a process for the production of ethylene from a feedstock comprising ethanol. Catalysts based on heteropolyacids are disclosed as being suitable for the dehydration of the ethanol feedstock.
WO 2008/138775 A1 discloses a process for the dehydration of one or more alcohols, which process comprises contacting one or more alcohols in the presence of one or more ethers with a supported heteropolyacid catalyst.
Before ethylene is used in downstream industrial applications, e.g. ethylene polymerization, the ethylene stream is typically subjected to a purification procedure. A primary impurity that is typically required to be removed from an ethylene stream is water, which is formed as a by-product in the dehydration of alcohols. For example, only a very low water content, typically in the order of low ppm concentrations, in an alkene feed, in particular an ethylene feed, is generally considered as acceptable for use in the preparation of polymers by catalytic polymerization; this is because water can act as a catalyst poison in the polymerization process. There are also other reasons why it is desirable to remove water from alkene, in particular ethylene, streams. For example, an ethylene stream may be utilised in downstream equipment which may be operated at very low temperatures, e.g. further purification of ethylene by distillation at, for example, −28° C.; at such low temperatures, any water present may freeze and could give rise to operational difficulties.
Different methods have been described in the prior art for the removal of water from ethylene. The use of molecular sieves is a known method for removing water (e.g. SRI Consulting (Menlo Park, Calif. 94025), Consulting Report PEP235, Process Economics Report 235, Chemicals from Ethanol, November 2007, describes that ethylene can be dried using a suitable desiccant such as a Type 3A molecular sieve); however, this method appears to be subject to several potential limitations and/or problems, such as:                it is a semi-batch operation,        ethylene may polymerise on the molecular sieve material,        it is very difficult to control the method to achieve the desired low levels of water content,        other organic components may be simultaneously caught on the molecular sieve material, and be either lost in regeneration or need expensive additional recovery methods, and        the molecular sieve separation method may cause a significant pressure drop which could increase the size of a downstream ethylene compressor, should one be required before the ethylene can be subjected to further downstream processing and/or use.        
Thus, there exists a need for alternative methods for the removal of water from an ethylene stream.